1. Field of Invention
This invention, a Radometer, relates to an improved method for the measurement of environmental radon and thoron in homes and buildings. More particularly, this invention allows for the accurate measurement of radon and thoron using a portable, handheld electrical instrument and only a few minutes of time.
2. Description of Related Art
The measurement of radon and thoron in homes and buildings is important in determining the presence of a potential health hazard to persons occupying the structure. The concentration of radon is measured by counting the alpha particles resulting from the radioactive decay of radon, .sup.222 Rn, while the level of thoron is determined by counting the alpha particles from the radioactive decay of thoron, .sup.220 Rn. In the present invention four counting channels are used: two for radon and two for thoron.
An electrostatic filtering system is employed to insure that no positively charged particles enter the sampling chamber. In prior art, the filters were either in the form of a physical filter such as an open pore foam or an electrostatic filter. The use of a physical filter greatly increases the power consumption necessary to measure either the radon or the thoron.
Prior art often used natural air flow or manual fanning devices to move the air being sampled into a sampling chamber and then to the sensor. To provide for a more accurate measurement, the subject invention employs a constant velocity pump which provides a constant flow rate for the air moving into the sampling chamber.
Humidity causes the charged radioactive progeny to become neutrally charged which results in a lowering of the sensitivity of the instrument. Attempts to compensate for the humidity using a linear compensation method do not yield satisfactory results. The Radometer employs a nonlinear equation as a means of compensation together with a desiccator as is needed.
The Radometer measures radon and thoron by electrostatic collection of charged progeny on a solid state detector. A dual electrostatic field technique eliminates the need for a physical air filter and electric power draining pumps present in other radon measuring instruments. The Radometer allows accurate measurements to be made in 10 to 15 minutes or less.
To perform radon measurements in under 15 minutes requires alpha spectroscopy which the Radometer employs. This method resolves and measures the 6 MeV alpha particle that is emmitted as the radon progeny of .sup.218 Po decay. Po-218 has a half life of 3 minutes which permits rapid measurement provided the instrument has adequate sensitivity. Using the dual electric field concept eliminates the need for filters or pumps since it insures that no electrically charged progeny can enter the measurement volume. This allows only the .sup.218 Po that collects on the solid state detector as a result of the decay of the radon in the collected gas sample to be measured.
Another problem that is commonly encountered occurs when making measurements in an area of low radon concentration following measurements taken in an area having a high concentration of radon. Generally in prior art, this change in environment causes the detector to have a high background making it difficult to distinguish low concentration values. The Radometer employs software that incorporates exponential superposition which allows for the rapid determination of low concentration values subsequent to a high concentration measurement.
Thus, one objective of this invention is to provide an instrument for the measurement of the concentration of radon and thoron in the environmental atmosphere in twenty minutes or less.
Another objective of this invention is to provide an instrument which is lightweight and provides for a long operation life using standard batteries which are readily available such as D-cell.
Another objective of the invention is to provide an instrument with the flexibility to operate in many different environments with little difficulty.
Additional advantages, objects and novel features of the invention will become apparent to those skilled in the art upon examination of the following and by practice of the invention.